Fix two TSAN bugs exposed when running with -c opt:

net/tools/quic/end_to_end_test.cc

Index: net/tools/quic/end_to_end_test.cc
diff --git a/net/tools/quic/end_to_end_test.cc b/net/tools/quic/end_to_end_test.cc
index 127fec715248cd5776f1a69ccd5233e2784b6d27..61ed2920b816f4b93ab84551662ed79159611946 100644
--- a/net/tools/quic/end_to_end_test.cc
+++ b/net/tools/quic/end_to_end_test.cc
@@ -1368,11 +1368,13 @@ TEST_P(EndToEndTest, NegotiateCongestionControl) {
       DLOG(FATAL) << "Unexpected congestion control tag";
   }
 
+  server_thread_->Pause();
   EXPECT_EQ(expected_congestion_control_type,
             QuicSentPacketManagerPeer::GetSendAlgorithm(
                 *static_cast<const QuicSentPacketManager*>(
                     GetSentPacketManagerFromFirstServerSession()))
                 ->GetCongestionControlType());
+  server_thread_->Resume();
 }
 
 TEST_P(EndToEndTest, LimitMaxOpenStreams) {